Download Water Chemistry - Biology12-Lum

Chemical Properties of Water - Part 1
©1998 by Alberts, Bray, Johnson, Lewis, Raff, Roberts, Walter
. http://www.essentialcellbiology.com
Published by Garland Publishing, a member of the Taylor & Francis Group.
HYDROGEN BONDS
d
Because they are polarized, two
adjacent H2O molecules can form
a linkage known as a hydrogen
bond. Hydrogen bonds have
only about 1/20 the strength
of a covalent bond.
d
+
H
_
2d
O
H
H
d
d
d
H
d
+
H
_
d
HYDROPHILIC MOLECULES
_
Na+
d
H
d
_
d
O
_
O
H
H
d
O
+
H
H
H
+
Hd
H
H
O
H
O
O_
H
+
H
O
+
_ d
Cl
d
H
H
H
H H
d
The cohesive nature of water is
responsible for many of its unusual
properties, such as high surface tension,
specific heat, and heat of vaporization.
HYDROPHOBIC MOLECULES
Substances that dissolve readily in water are termed hydrophilic. They are
composed of ions or polar molecules that attract water molecules through
electrical charge effects. Water molecules surround each ion or polar molecule
on the surface of a solid substance and carry it into solution.
Od
0.10 nm
covalent bond
Molecules of water join together transiently
in a hydrogen-bonded lattice. Even at 37oC,
15% of the water molecules are joined to
four others in a short-lived assembly known
as a “flickering cluster.”
Although a water molecule has an overall neutral charge (having the same
number of electrons and protons), the electrons are asymmetrically distributed,
which makes the molecule polar. The oxygen nucleus draws electrons away
from the hydrogen nuclei, leaving these nuclei with a small net positive charge.
The excess of electron density on the oxygen atom creates weakly negative
regions at the other two corners of an imaginary tetrahedron.
H
O
electropositive
region
_
electronegative
region
_
H
+
O
d
O
WATER STRUCTURE
H
H
_
hydrogen bond
WATER
O
2d
+
Two atoms, connected by a covalent bond, may exert different attractions for
the electrons of the bond. In such cases the bond is polar, with one end
_
slightly negatively charged (d ) and the other slightly positively charged (d +).
H
hydrogen bond
0.27 nm
H
+
d
O
Hydrogen bonds are strongest when
the three atoms lie in a straight line.
bond lengths
+
H
H
H
O
H
H
+
O
O
O
H
O
H
O
N
H
O
H
H
H
H
H
H
C
H
H
H
Ionic substances such as sodium chloride
dissolve because water molecules are
attracted to the positive (Na+) or negative
_
(Cl ) charge of each ion.
H
N
C
H
Molecules that contain a preponderance of nonpolar bonds are usually insoluble in water and are
termed hydrophobic. This is true, especially, of
hydrocarbons, which contain many C–H bonds.
Water molecules are not attracted to such
molecules and so have little tendency to surround
them and carry them into solution.
C
O
H
H
Polar substances such as urea
dissolve because their molecules
form hydrogen bonds with the
surrounding water molecules
H
H
O
H
H
H
O
H
H
H
O
H
H
C
H
O
O
H
H
O
H
O
H
H
Chemical Properties of Water - Part 2
©1998 by Alberts, Bray, Johnson, Lewis, Raff, Roberts, Walter
. http://www.essentialcellbiology.com
Published by Garland Publishing, a member of the Taylor & Francis Group.
WATER AS A SOLVENT
Many substances, such as household sugar, dissolve in water. That is, their
molecules separate from each other, each becoming surrounded by water molecules.
When a substance dissolves in a
liquid, the mixture is termed a solution.
The dissolved substance (in this case
sugar) is the solute, and the liquid that
does the dissolving (in this case water)
is the solvent. Water is an excellent
solvent for many substances because
of its polar bonds.
sugar
dissolves
water
molecule
sugar crystal
sugar molecule
HYDROGEN ION EXCHANGE
ACIDS
Positively charged hydrogen ions (H+) can spontaneously
move from one water molecule to another, thereby creating
two ionic species.
H
H
H
H
+
O H O
O H + O
Substances that release hydrogen ions into solution
are called acids.
HCl
H+
hydrochloric acid
(strong acid)
hydrogen ion
+
Cl–
chloride ion
H
Many of the acids important in the cell are only partially
dissociated, and they are therefore weak acids—for example,
the carboxyl group (–COOH), which dissociates to give a
hydrogen ion in solution
O
+
C
(weak acid)
hydroxyl
ion
BASES
pH
+
H
conc.
moles/liter
ACIDIC
_
2
10
3
10
10
_3
_4
_5
_6
_
10 7
_
10 8
ALKALINE
pH
10 1
_
10 2
10
pH = _log10[H+]
_
[H+] = 10 7 moles/liter
OH–
+
Since the process is rapidly reversible, hydrogen ions are
continually shuttling between water molecules. Pure water
contains a steady state concentration of hydrogen ions and
hydroxyl ions (both 10–7 M).
Note that this is a reversible reaction.
For pure water
H+
H2 O
hydrogen
ion
O–
OH
The acidity of a
solution is defined
by the concentration
of H+ ions it possesses.
For convenience we
use the pH scale, where
often written as:
O
H+
C
H
hydronium ion
hydroxyl ion
(water acting as (water acting as
a weak base)
a weak acid)
10
10
10
10
10
10
_9
_10
_11
_12
_13
_14
1
4
5
6
7
8
9
10
11
12
13
14
Substances that reduce the number of hydrogen ions in
solution are called bases. Some bases, such as ammonia,
combine directly with hydrogen ions.
NH3
ammonia
+
H+
NH4+
hydrogen ion
ammonium ion
Other bases, such as sodium hydroxide, reduce the number of
H+ ions indirectly, by making OH– ions that then combine
directly with H+ ions to make H2O.
Na+
NaOH
sodium hydroxide
(strong base)
sodium
ion
+
OH–
hydroxyl
ion
Many bases found in cells are partially dissociated and are termed
weak bases. This is true of compounds that contain an amino
group (–NH2), which has a weak tendency to reversibly accept an
H+ ion from water, increasing the quantity of free OH– ions.
–NH2
+
H+
–NH3+